1. Field of the Invention
The present invention relates to kneading apparatuses. More particularly, the invention relates to a kneading apparatus used for masticating rubber and for kneading a rubber-based composition in which rubber and various compounding ingredients are mixed, and to a kneading method using the kneading apparatus.
2. Description of the Related Art
In order to produce a kneaded rubber-based composition including rubber and various compounding ingredients, a batch process is often used, in which predetermined amounts of raw materials are intermittently kneaded. However, in order to improve productivity, a method for continuously kneading a rubber-based composition has also been disclosed (Japanese Unexamined Patent Application Publication No. 11-262945). In such a method, a twin-screw extruder, which is a typical kneader for rubber-based compositions, is used. The twin-screw extruder is provided with a rubber feed port, and a rubber-feeding extruder is further connected thereto. Continuous kneading is performed by the twin-screw extruder while continuously feeding the rubber-based composition.
However, the continuous kneading method according to Japanese Unexamined Patent Application Publication No. 11-262945 is not suitable for rubber-based compositions having high viscosities, and as described in Example 1 in the patent application publication, the method aims to continuously knead a rubber-based composition having a low viscosity, e.g., a Mooney viscosity (100° C.) of approximately 47.
When a rubber-based composition having a high viscosity, e.g., a Mooney viscosity (100° C.) of more than 100, is kneaded, such as in the case of a composition in which natural rubber as a major ingredient and 30 parts or more of carbon are mixed, kneading treatment is performed to improve the dispersion of various compounding ingredients and also to decrease the viscosity of the rubber-based composition to a viscosity suitable for later processes, such as extrusion molding. The viscosity is decreased because rubber molecular chains are cut due to mechanical shearing. However, heat generation due to mechanical shearing forces applied to rubber increases as the viscosity is increased, and if the temperature becomes excessively high, the physical properties of the rubber are changed, thereby degrading the performance of the rubber. In general, if the temperature exceeds 160° C., the performance of the rubber is hindered. Because of the high viscosity of rubber, the cutting effect of molecular chains due to mechanical shearing is easily demonstrated when kneading is performed at lower temperatures, which is also advantageous in terms of kneading efficiency. In practice, when a rubber-based composition with a high viscosity is kneaded, it is extremely difficult to achieve a predetermined decrease in viscosity and a predetermined degree of dispersion of the compounding ingredients while the materials to be kneaded are maintained at low temperatures so as not to exceed the temperature range required to ensure the physical properties of the rubber. Therefore, kneading is performed by a batch-type kneader. When the temperature reaches approximately 160° C. during kneading, the materials to be kneaded are recovered from the kneader and cooled to ambient temperature after sheet forming is performed. Kneading is performed again in order to decrease the viscosity. Such a rekneading step is referred to as a remill step.
However, when a rubber-based composition having a high viscosity is kneaded by the method described above, the remill step is usually repeated several times until the predetermined viscosity is achieved. Remilling is often performed approximately five times. Consequently, the productivity inevitably decreases due to repeated kneading by the batch-type kneader and cooling, and since thermal hysteresis then takes place, alteration of the rubber easily occurs.